The synthesis of organosilicon compounds by reaction of an organic compound containing an alkenyl group with a hydrosilane, resulting in addition of silicon and hydrogen across the carbon-carbon double bond of the alkenyl group, is well known. The general method, usually carried out in the presence of a transition metal catalyst, has been known for many decades. It has generally been believed that continuous processes are not suitable for the production of acryloyl- or methacryloyl-organosilicon compounds using hydrosilation catalysts, for a number of different reasons. First, it would be expected that polymerization would occur and would disrupt the flow process; second, an induction period is required for catalyst activation, and this would be expected to mean that a continuous process would not work; and third, deactivation of the catalyst is known to occur over time. U.S. Pat. No. 5,359,113 describes this latter problem, which it believes is caused by lack of oxygen, and solves the problem by adding a peroxide into the hydrosilylation reaction. Using MeHSiCl2 as the silane and allyl chloride as the unsaturated compound, Comparative Example 1 of U.S. Pat. No. 5,359,113 illustrates catalyst deactivation during a continuous reaction process, while Example 1 shows that addition of a peroxide avoids this problem, permitting the use of a continuous reaction process. However, the reaction of U.S. Pat. No. 5,359,113 cannot in practice be used for the preparation of acryloyl- or methacryloylsilicon compounds because peroxide is a known initiator for polymerization. Even without the addition of peroxide, it is known that polymerization causes major problems in known processes for the preparation of acryloyl- or methacryloylsilicon compounds.
3-Methacryloxypropyl-bisdimethylchlorosilane (MOPDMCS) is a key raw material for the manufacture of methacryloxy propyl dimethyl methoxysilane (mPDMS), which is used in the manufacture of contact lenses. MOPDMCS is manufactured by the reaction of allyl methacrylate (AMA) and dimethylchlorosilane (DMCS) in the presence of a transition metal hydrosilylation catalyst, usually based on platinum.
However, known processes for the preparation of MOPDMCS suffer from a number of disadvantages, MOPDMCS being a particularly difficult methacryloyl organosilicon compound to prepare and handle. Controlling the process is difficult, and it is difficult to ensure batch-to-batch consistency. As a result, problems arise, in particular relating to product consistency and purity. For example, hydrolysis and dimerisation of the product occurs readily. Further, the product may form homopolymers, or it may copolymerize with by-products present in the reaction mixture. Such reactions can occur at any time during the preparative process and also during purification of the product, which may for example be carried out by distillation. It is thus difficult to prepare MOPDMCS having a high degree of purity.
Much research has been carried out to develop methods which reduce degradation of the product during or after preparation, either by reducing the content of by-products obtained, or by reducing the dimerization or polymerisation of the product. For example, U.S. Pat. No. 5,550,272 and EP 803507 describe the use of free radical polymerization inhibitors. U.S. Pat. No. 5,847,178 proposes adding copper compounds to the reaction mixture. EP 775,708 describes treatment of the reaction mixture with an inorganic chloride, while U.S. Pat. No. 5,811,565 describes treatment of the reaction mixture with a Lewis acid. EP 693492 describes carrying out the synthesis under low water conditions to minimize polymerization. EP 562584, WO 2004/085446 and EP 753521 describe additives to be used during purification of the product by distillation, aiming to reduce polymerization.
These known processes are all carried out using conventional batch processing techniques, and none of them addresses one major problem which contributes to batch-to-batch inconsistency, which is that the reaction has a long induction time during which activation of the catalyst takes place. Not only is the induction time long, but it is unpredictable, varying significantly from batch to batch. Generally, if reactants are added during the induction period, they will accumulate, and once the reaction begins, large quantities of heat are generated and the temperature will rise to an undesired level. Under these circumstances, runaway reactions and polymerization are likely to occur. Even if steps are taken to monitor and deal effectively with the induction period, batch-to-batch consistency tends to be low.
US 2010/179340 describes some of the disadvantages of batch processes for the preparation of methacryloyloxypropylalkoxysilanes, and describes a continuous process for the preparation of methacryloylorganosilicon compounds by the reaction of allyl methacrylate with a hydrosilane using a specific system of reactors. The hydrosilanes used in the process are alkoxyhydrosilanes. WO 2008/017555 contains a very similar disclosure to US 2010/0179340, and as for US 2010/0179340, the example of preparing a methacryloxy compound is carried out using a methoxysilane.
Methacryloylalkoxysilanes of the type prepared in US 2010/0179340 and WO 2008/017555, and the alkoxyhydrosilanes used to make them, are known to be relatively stable and easy to handle compared with other hydrosilanes. Specifically, a reaction mixture including AMA, a platinum hydrosilylation catalyst and DMCS would be expected to have a much greater propensity to form by-products and/or polymerise during synthesis than the corresponding reaction mixture including trimethoxysilane. MOPDMCS is particularly difficult to synthesise, due to its propensity to polymerise. It is believed that this is because Si—Cl bonds are highly susceptible to hydrolysis leading to the formation of hydrochloric acid, which catalyses both further decomposition and polymerization of any methacrylate bonds present. Because of its instability, MOPDMCS is always sold containing a significant level of polymerization inhibitors, typically from 100-200 ppm BHT plus 200-400 ppm MeHQ. In contrast, the corresponding 3-methacryloxypropyltrimethoxysilane (MOPTMOS) is sold with a very low level of polymerization inhibitor, typically less than 10 ppm BHT. The skilled person would not have expected that a continuous process for the preparation of MOPDMCS could be carried out successfully.
There remains a need for improved processes for the production of MOPDMCS. We have now found a highly efficient process which permits their preparation with high yield, as well as a high degree of consistency and purity. This is particularly surprising given the highly reactive nature of the reactants and product, and in particular their propensity to react with themselves and each other.